sclass TreeHasKeyMap<A, B extends HasKey<A>> extends HasKeyMap<A, B> {
  *() { init(); }
  *(Iterable<B> l) { init(); addAll(l); }
   
  void init { map = new TreeMap; }
  
  TreeMap<A, B> map() { ret (TreeMap) map; }
  
  A lastKey() { ret main lastKey(map()); }
  
  B higher(B a) { ret a == null ?: higherValue(map(), a.getKey()); }
}

Began life as a copy of #1032972